There are conventional electronic circuits that are used to produce electrical signals with frequencies up to the gigahertz range, but it is much more difficult to produce higher frequencies. This is achieved in the domain of microwaves with Gunn diodes or Impatt diodes, up to about 100 gigahertz but with powers that decrease as the frequency increases. Frequencies in the ‘terahertz domain’, i.e. frequencies from 100 gigahertz to 10 terahertz (at the limit of very far infrared) cannot be effectively obtained through purely electronic circuits.
Optical or electronic signals in the terahertz domain would, however, be useful, e.g. for imaging (infrared or visible opaque media imaging), or for broadband telecommunications through the atmosphere (at frequencies not undergoing too much atmospheric absorption), or for the spectrometry of certain molecules.
It has been suggested that very high frequencies may be produced through the difference between the frequencies of electromagnetic light waves. Here light waves will be understood to mean waves in a wavelength range covering not only visible, but also infrared and ultraviolet light. To give an idea of the order of magnitude: an infrared laser beam with a wavelength of 1 micron corresponds to a light frequency of approximately 300 terahertz (300×1012 hertz). If two light beams with frequencies of 300 and 301 terahertz are mixed together, a 1 terahertz subtractive beat is obtained. Thus a 1 terahertz amplitude-modulated light beam can be produced, and this beam can be used either in optical form or in the form of an electronic signal by conversion in a fast photodiode or a photoswitch.
But the difficulty is that the signal produced by beating two light frequencies has a frequency that is extremely dependent on the stability of these two starting frequencies. These frequencies are produced by monochromatic lasers, but the lasers are not naturally sufficiently stable. They have a significant frequency noise or phase noise.